Below is a map that shows some of these earthquakes with their moment tensors.

I placed a moment tensor / focal mechanism legend on the poster. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely.

I include some inset figures in the poster.

In the upper right corner is a tectonic map showing the plate boundaries (Richards et al., 2011).

To the left of the Richards et al. (2011) figure is a cross section of a generic ocean-ocean subduction zone, where oceanic lithosphere is subducting beneath oceanic lithosphere. Note the location of the magmatic island arc. Vanuatu and the other islands in this region are formed as part of an island arc.

In the lower left corner I place a subset of the USGS tectonic map poster for this region of the Pacific Ocean. This is from the USGS Open File Report 2010-1083-I (Benz et al., 2011). Hypocenters are plotted as cross sections to show the geometry of the subducting slabs.

This map is larger scale, showing the slab contours. The red-orange-yellow lines are slab contour lines from Hayes et al. (2012). These lines are a best estimate for the depth to the subduction zone fault. These are based largely upon seismicity and there is currently an effort to update these contours to integrate other data types. The hypocentral depth is 35 km, and the slab contour nearest the epicenter is 40 km (so this makes sense that it is associated with the subduction zone megathrust).

I include some inset figures in the poster.

In the lower left corner, I include a map from Cleveland et al. (2011). This figure shows earthquakes and some moment tensors for larger earthquakes.

In the upper right corner I place a subset of the USGS tectonic map poster for this region of the Pacific Ocean. This is from the USGS Open File Report 2010-1083-I (Benz et al., 2011). Hypocenters are plotted as cross sections to show the geometry of the subducting slabs.

Here are some animations from IRIS that show seismic waves propagating through the USArray, a suite of seismometers in the USA and parts of Canada. Above is a screenshot and below is a video.

Here is an animation that shows the seismicity for this region from 1960 – 2016 for earthquakes with magnitudes greater than or equal to 7.0.

I include some figures mentioned in the posters above, in addition to a plot from Cleveland et al. (2014). In the upper right corner, Cleveland et al. (2014) on the left plot a map showing earthquake epicenters for the time period listed below the plot on the right. On the right is a plot of earthquakes (diameter = magnitude) of earthquakes with latitude on the vertical axis and time on the horizontal axis. Cleveland et al (2014) discuss these short periods of seismicity that span a certain range of fault length along the New Hebrides Trench in this area. Above is a screen shot image and below is the video.

(left) Seismicity of the northern Vanuatu subduction zone, displaying all USGS-NEIC earthquake hypocenters since 1973. The Australian plate subducts beneath the Pacific in nearly trench-orthogonal convergence along the Vanuatu subduction zone. The largest events are displayed with dotted outlines of the magnitude-scaled circle. Convergence rates are calculated using the MORVEL model for Australia Plate relative to Pacific Plate [DeMets et al., 2010]. (right) All GCMT moment tensor solutions and centroids for Mw ≥ 5 since 1976, scaled with moment. This region experiences abundant moderate and large earthquakes but lacks any events with Mw >8 since at least 1900.

Figure 17. I include the figure caption below as a blockquote.

One hundred day aftershock distributions of all earthquakes listed in the ISC catalog for the 1966 sequence and in the USGS-NEIC catalog for the 1980, 1997, 2009, and 2013 sequences in northern Vanuatu. The 1966 main shocks are plotted at locations listed by Tajima et al. [1990]. Events of the 1997 and 2009 sequences were relocated using the double difference method [Waldhauser and Ellsworth, 2000] for P wave first arrivals based on EDR picks. The event symbol areas are scaled relative to the earthquake magnitudes based on a method developed by Utsu and Seki [1954]. Hypocenters of most aftershock events occurred at <50 km depth.

Figure 17. I include the figure caption below as a blockquote.

(right) Space-time plot of shallow (≤ 70 km) seismicity M ≥ 5.0 in northern Vanuatu recorded in the NEIC catalog as a function of distance south of 10°N, 165.25°E. (left) The location of the seismicity on a map rotated to orient the trench vertically.

Here are some plots from Jascha Polet showing focal mechanisms for this subduction zone. The top figure is a map and the bottom figure is a cross section.

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Status & Contact Info

Jay Patton is currently living in Arcata, CA. He was most recently teaching classes at Humboldt State University, Dept. of Geology. and is available for geological consulting.
HSU Dept. Geology website twitter: @patton_cascadia
Contact him at:
quakejay at gmail.com

Chicken the Cat

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